Milk is one of the most popular items in a supermarket. And yet, it is often found at the very back of the store. One theory is that stores want you to walk further through the aisles so you'll buy more stuff. But there's another explanation that's more innocent. David Kestenbaum with our Planet Money team tried to find out which is right.

DAVID KESTENBAUM, BYLINE: To lay out the two theories, we have two - I'm going to call them milk theorists. First - food writer Michael Pollan, author, most recently, of "Cooked: A Natural History of Transformation." Michael gets his milk at a small grocery store in Berkeley, California.

MICHAEL POLLAN: I get a local milk called san benois which is not homogenized but pasteurized and is whole.

KESTENBAUM: Second milk theorist - economist Russ Roberts at the Hoover Institution. He shops at Costco. For the record - not a milk drinker.

KESTENBAUM: So you're disqualified from this conversation as being lactose intolerant.

ROBERTS: Well, as an academic, I think I'm more qualified by the fact that I don't, you know, know much about it.

KESTENBAUM: Michael Pollan says the idea that stores put milk in the back to try to get you to buy more stuff makes sense to him.

POLLAN: My general impression is that the milk is in the back, but it's also usually very far from the bread. Both of them are very common items, and so it makes you cover a lot of ground if you want them.

KESTENBAUM: Our other milk theorist Russ Roberts does not buy this idea. In a competitive environment, he says, you can't really get away with manipulating your customer too much because they'll just go to a different store. Russ thinks stores keep milk in the back because it's easier. The trucks deliver the milk to the back. There's a big refrigerated room there. If hiding the stuff people want in the back of your store was really a good business strategy, he says, why don't other stores do it? Why don't bookstores put the best-sellers in the back? They don't.

ROBERTS: There in the front. They don't make you go through the Elizabethan poetry - not that we wouldn't be better people for it if we had to go through that section - but they tend to put it in the front to make it easy.

KESTENBAUM: All right, so Michael, what do you think of his argument that it's there to serve the customer?

POLLAN: Well, I think Russ is assuming a greater degree of rationality in the American consumer than the designers of supermarkets assume. I mean, they're trying to work on us at subliminal levels.

KESTENBAUM: These are two very different worldviews. Russ Roberts walks into a store and thinks, this is great. Look how well they're serving me. Michael Pollan walks into a supermarket and is suspicious. So why is the milk in the back? I tried to find out. I went and interviewed a bunch of supermarkets and consultants. Rob Olsen is dairy buyer for BJ's Wholesale Club, a major competitor to Costco and Sam's Club. I asked why the milk was in the back.

ROB OLSEN: I believe we want members or customers to get to the back of the club and see the wide array of items that we carry.

KESTENBAUM: So the idea is it gets them into the store, gets them to look at the other stuff and say, oh maybe I need that, too.

OLSEN: Correct. We call it building the basket in the business.

KESTENBAUM: Building the basket - seemed like a point for Michael Pollan. But then he mentioned another phrase - the cold chain. Once milk from a cow is put in a gallon jug, you want it cold every step of the way.

OLSEN: Brought into a cold room and it's kept in this cold room. It goes from the cold room to the cold truck to the cold distribution center back on a cold truck to the club or store.

KESTENBAUM: The PR person for BJ's later assured me this was the primary reason, but both ideas are clearly out there. A spokesman for the supermarket chain Kroger mentioned both reasons, and here's Burt Flickinger, a well-known supermarket consultant.

BURT FLICKINGER: I tell the retailers to put the milk in the front of the store for the convenience of the customers, and the retailers, typically, ignore my advice and put the milk in the back of the store where they've been putting it for 70 to 80 years.

KESTENBAUM: So they say it's for convenience sake - cheaper, less labor?

FLICKINGER: All of the above.

KESTENBAUM: They don't say it's so that people have to walk through our store and see other things they might want to buy.

FLICKINGER: And then they say we want the people to walk through the stores and buy everything we sell.

KESTENBAUM: I think we have this idea in economics that where there is a question, there is an answer. But sometimes there just isn't. No one I talked to had a big binder on their desk titled optimum milk placement, an empirical study. In the end, the score between Michael Polland and Russ Roberts was about even. Should we call this one a draw?

ROBERTS: Yeah, sure

POLLAN: I'm happy to do that, sure.

KESTENBAUM: I think if someone asks you, is the milk back in the back of the store because it's easier or because it makes you walk through the store, a safe answer is just to say yes. David Kestenbaum, NPR News. Transcript provided by NPR, Copyright NPR.